Microminiature field emitters are well known in the microelectronics art. These microminiature field emitters are finding widespread use as electron sources in microelectronic devices. For example, field emitters may be used as a source of electrons in electron guns employed in flat panel displays for use in aviation, automobiles, workstations, laptops, head mounted displays, heads up displays, outdoor signage, or practically any application for a screen which conveys information through light emission. Field emitters may also be used in non-display applications such as power supplies, printers, and X-ray sensors.
When used in a display, the electrons emitted by a field emitter are directed to a cathodoluminescent material. These display devices are commonly called Field Emission Displays (FEDs). A field emitter used in a display may include a microelectronic emission surface, also referred to as a "tip" or "microtip", to enhance electron emissions. Conical, pyramidal, curved and linear pointed tips are often used. Alternatively, a flat tip of low work function material may be provided. An extraction electrode or "gate" may be provided adjacent, but not touching, the field emission tip, forming a field emission gap. Upon application of an appropriate voltage between the emitting electrode and the gate, quantum mechanical tunneling, or other known phenomena, cause the tip to emit electrons. Once emitted, the electrons are predominately affected by the electric field between a highly charged anode and the field emitter. The electrons are accelerated toward the anode.
In microelectronic applications, an array of field emission tips may be formed on the horizontal face of a substrate such as a silicon semiconductor substrate. Emitting electrodes, gates and other electrodes may also be provided on or in the substrate as necessary. Support circuitry may also be fabricated on or in the substrate.
The FEDs may be constructed using various techniques and materials, which are only now being perfected. Preferred FED's may be constructed of semiconductor materials, such as silicon. There are two predominant processes for making field emitters: the "wells first" processes; and the "tips first" processes. In wells first processes, such as a Spindt-type process, the wells are first formed in a material, and tips are later formed in the wells. In the tips first processes, the tips are formed first, and the wells are formed around the tips. There are multitudes of variations of both the wells first and the tips first processes. The present invention is equally applicable to field emitters made by any processes, whether it be wells first, tips first, or some other process.
The electrical theory underlying the operation of an FED is similar to that for a conventional CRT. Electrons emitted from the tips are accelerated by the anode in the direction of an opposing display surface. These high energy electrons strike phosphors on the inside of the display and excite them to luminesce. An image is produced by the pattern of luminescing phosphor pixels as viewed on the display screen by an observer. This process is a very efficient way of generating a lighted image.
In a CRT, one or three electron guns are provided to generate all of the electrons which impinge on the display screen. A complex electron deflection device, usually comprising power-consuming electromagnets, is required in a CRT to direct the electron stream towards the desired pixels. The combination of the electron gun and deflection device behind the screen necessarily make a CRT display relatively thick.
FEDs, on the other hand, may be relatively thin. Each pixel of an FED has its own electron source, typically an array or grouping of emitting microtips, which may share a common conductive base pad. The electric field between the cathode and the gate causes electrons to be emitted from the microtips. FEDs are thin because the microtips and gates, which are the equivalent of an electron gun in a CRT, are extremely small. Further, an FED does not require a deflection device, because each pixel has its own electron gun (i.e. gate and emitters) positioned directly behind it. The emitters need only be capable of emitting electrons in a direction generally normal to the FED substrate and toward the anode.
Because of the unique benefits and numerous potential applications for FEDs, research and development on field emitter devices is extremely active. Numerous problems however, continue to plague the production of suitable emitter devices. These problems include flashover control, electron divergence control, film stress minimization, satisfactory emitter tip formation, and flexible emitter device fabrication.
Preventing flashover is an important goal of FED development efforts. Flashovers are discharges between adjacent gates, between emitter tips and gates, and even between gates and the anode. Though the space between the emitter and the anode may typically be evacuated in an FED, the materials that make up the FED are likely to outgas over time. Outgassing is caused by adsorbed and absorbed gas molecules in the glass and metal structures of the FED that escape into the vacuum space. These gas molecules may become ionized as a result of being bombarded by the electrons emitted from the field emitter tips. The ionized gas molecules may provide an electrical path for flashovers. In FEDs in which the potential between the anode and the cathodes is in the range of thousands of volts, such flashovers may be catastrophic to the device. Even if the flashover is not initially catastrophic, flashover may vaporize materials in the FED, resulting in the production of additional gas molecules, and sowing the seeds for future flashover.
Another problem which has been encountered in the operation of FEDs is the scattering of electrons emitted from the tips. Under the pull of the strongly positively charged anode, emitted electrons preferably impinge on a phosphor particle located directly opposite the tip from which the electron is emitted. Due to the influence of laterally spaced gates, however, some portion of the emitted electrons may deviate slightly from a straight pathway. There may be some horizontal dispersion of electrons by the time they reach the phosphor layer. This dispersion increases pixel size impinges on adjacent pixels, reducing the resolution of the display.
Both the risk of flashover, as well as the horizontal dispersion of emitted electron may be reduced through the use of lower gate voltages. The magnitude of gate voltage necessary to obtain a desired electron emission current is dependent upon the inherent work function of the emitter tip material as well as the surface curvature of the emitter. This dependency may be understood during an examination of the physics of field emission of electrons. As is well known in the art, field emission of electrons may be explained by Fowler-Nordheim, or quantam-mechanical, tunneling. Normally, electrons cannot escape from a metal unless the metal is heated. However, under sufficiently high applied fields, electrons can tunnel through the barrier, even at low temperatures. The rate of the electron field emission is dependent upon the material's inherent work function and the applied field which can be enhanced by the surface curvature of the emitter.
Thus, it is advantageous to use low work function materials for emitter tips. This permits lower gate voltages. Lower gate voltages reduce the risk of flashover. They also decrease the horizontal dispersion of emitted electrons. Other benefits of lower gate voltages include: lower cost cathode and gate driver circuitry, simplified gray scale implementation, reduced power loss in charging cathode and gate lines, and faster devices for non-display applications. Voltages below 25 volts result in additional benefits, such as the opportunity to use inexpensive MOS drivers, the elimination of ion sputtering as a cause of emission decay, and improved pixel spot size control due to reduced lateral electron energy and, hence, reduced horizontal dispersion.
Emitter tips may be formed by a variety of processes well known in the art such as molding, etching or evaporation. The emitter materials may be deposited by various processes, including evaporative deposition, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and sputtering processes. Metal emitter tips are typically formed by evaporated deposition. Whatever formation method is employed, it is important that the chosen method applied to the given material results in an emitter tip with a high surface curvature. Likewise, it is important that the emitter tip material exhibit low surface mobility to prevent dull emitter tip profiles.
When the evaporative tip formation technique is used to form the emitter tip (cone), it is also desirable that the emitter materials have a moderately high aspect ratio (the ratio of the base of the tip to its height). Too small a ratio limits the gate insulator thickness for any given gate hole diameter. Too large a ratio demands an excessively thick insulation for any given hole diameter. Excessively thick insulation is difficult to fabricate and to clean out after subsequent processing. Values of aspect ratio between 1 and 2 produce a good compromise between gate hole diameter and insulation thickness.
Unfortunately, many materials previously tried as emitter tips have either not formed a high aspect ratio tip, or have high surface mobility leading to dulling and rounding of the tip. For example, poor materials include titanium, aluminum and nickel.
The challenge of making a viable FED does not end when a suitable low work function material has been formed into a high aspect ratio tip. A perennial problem has been the formation of non-conductive oxides on the emitter tips. Vacuum sealing operations used to make FEDs often involve high temperature processing. Slight partial pressures of oxygen or moisture may be present in an imperfect vacuum, allowing oxides to form on the emitter tip material. These oxides are often non-conductive, or at a minimum, less conductive than the pure emitter tip material. The result is a higher emitter tip work function, necessitating the use of higher gate voltages, with the concomitant deleterious effects herein previously discussed.
Many materials and methods have been tried to overcome this tip oxidation problem. Itoh et al., U.S. Pat. No. 5,469,014 (Nov. 21, 1995), for a "Field Emission Element," for example, teaches an electron emitter having a base and a tip portion wherein the base is made of Ti or Cr, while the tip is made of Ta, Nb, TiN, T, C or Mo. The tip exhibits low oxygen bonding strength, while the base has a higher oxygen bonding strength. As a result, atoms and/or molecules entering the tip or base are absorbed on the base material without forming any oxide layer on the tip.
Another problem experienced by field emitter tips is nonuniform or excessive current. Current production is exponential with the electric field applied across the tip. The field is a function of gate voltage, gate to tip distance, and tip sharpness. Small variations in tip sharpness, for example, can produce significant variations in current. These variations are exhibited as nonuniformity in screen brightness, and if excessive, may explosively melt emitter tips causing defects. Current limiters are used to control current production.
Current-limiting materials are often employed in the layers below the emitter tips to limit the current exiting each of the emitters. In devices employing cathodes spaced laterally from emitter tips, with current-limiting material disposed therebetween, a conductive pad may be used as a base for emitter tips within a pixel to equalize the potential at each of the tips. Though such devices exhibit beneficial current limiting properties, as well as increased device stability, emitter-to-emitter electron emission uniformity within a pixel may still be less than desirable. Emission nonuniformity among emitters within a pixel may be due to current stealing by emitters which have lower effective resistivity because of variations in composition and shape.
The evaporative deposition techniques by which metal emitter tips are often formed, such as in a Spindt-type emitter fabrication process, involve difficulties which have not been always satisfactorily resolved. Some materials exhibit compositional changes during evaporation and deposition, resulting in unpredictable emitter performance.
The evaporative deposition process occurs at temperatures at or above the evaporation temperature of a given emitter tip material. Refractory materials such as molybdenum, a commonly used emitter tip material, require relatively high temperatures such as, for example, greater than 3000.degree. C. These materials are often difficult to evaporate and are, therefore, not ideal. Thus, there is still a need for materials which minimize these evaporative deposition problems.
Another problem confronting designers of FEDs is maintaining flexibility in selection and ordering of fabrication process stages. FEDs are commonly formed by semiconductor manufacturing processes which are well known in the art. These processes often include fabrication steps which entail exposure of the emitter tip material to various acids and bases. Unless the emitter tip material is relatively chemically inert in the presence of these acids and bases, fabrication process options are limited. For example, acids and bases can provide electrolytes which either etch emitter tips or coat emitter tips, degrading their performance.
Minimizing film stresses is another challenge facing the FED industry. FEDs are commonly constructed using glass substrates as foundations for a series of film layers in accordance with well known semiconductor fabrication layering techniques. High film stresses may develop within the thin layers of these devices. Film stresses are undesirable because cracks may develop in these layers and may propagate to other layers of the structure causing broken conductor pathways or shorts.
Various materials have been tried as emitter tips in the hope of solving these problems and achieving these sometimes mutually inconsistent goals. Materials that have been tried include Ti and Al. Materials such as pure Cr, for example, are not suitable for electron emitter tips because of their inherently high film stress and its tendency to form a difficult to remove oxide surface. Most cermets exhibit unduly high resistivity. In sum, each material previously employed as an emitter tip has certain drawbacks.
What is needed is a high performance emitter tip material which can be formed into high aspect ratio, low work function tips which maintain their shape, thus minimizing flashover risks and electron scattering problems, and which at the same time permits a high level of fabrication process flexibility, while minimizing film stresses.